fix: Fix BadMutable plugin bug in .diff (#1023)

Author: iamkun

src/utils.js

@@ -18,9 +18,9 @@ const monthDiff = (a, b) => {
   // function from moment.js in order to keep the same result
   if (a.date() < b.date()) return -monthDiff(b, a)
   const wholeMonthDiff = ((b.year() - a.year()) * 12) + (b.month() - a.month())
-  const anchor = a.add(wholeMonthDiff, C.M)
+  const anchor = a.clone().add(wholeMonthDiff, C.M)
   const c = b - anchor < 0
-  const anchor2 = a.add(wholeMonthDiff + (c ? -1 : 1), C.M)
+  const anchor2 = a.clone().add(wholeMonthDiff + (c ? -1 : 1), C.M)
   return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
     (anchor2 - anchor)))) || 0)
 }

test/plugin/badMutable.test.js

@@ -164,6 +164,16 @@ it('Locale', () => {
   expect(d.format(format)).toBe(m.format(format))
 })
 
+it('Diff', () => {
+  const d = dayjs()
+  const m = moment()
+  const unit = 'year'
+  const d2 = d.clone().add(1, unit)
+  const m2 = m.clone().add(1, unit)
+  expect(d.diff(d2, unit)).toBe(-1)
+  expect(m.diff(m2, unit)).toBe(-1)
+})
+
 it('isAfter isBefore isSame', () => {
   const d = dayjs()
   const format = dayjs().format()